Color television standards have progressed with an emphasis on backwards compatibility to ensure new standards will function on older black and white televisions. As a result, modern color television standards call for a basic black and white brightness signal, called luminance (luma), as well as a color signal, called chrominance (chroma). Current color television encoding processes (e.g. NTSC, PAL and SECAM) provide this color and brightness information by summing up the luma and chroma signals in an overlay frequency spectrum and results in a so-called composite signal. The video decoding system must separate the luma and chroma signals from the composite signal and decode these signals to obtain the video components.
In a SECAM based system, the chroma information is modulated on a subcarrier through frequency modulation, and luma information is carried in the baseband with spectrum beyond the chroma's subcarrier. In the receiving system, the luma and chroma are often separated using band-trap filter and band-pass filters. A band-trap filter is used to suppress the chroma subcarrier to capture the luma spectrum, and the band-pass filter is used to capture the chroma spectrum. The quality of the image derived from the luma and chroma spectrums is dependent on the bandwidths of both the band-trap filter and the band-pass filters. For example in separating luma from the composite video, if the trap band of the band-trap filter is narrow, a portion of the chroma signal will appear in the luma signal, his result is known in the art as dot crawl; if the trap band of the band-trap filter is wide, the luma will lose sharpness. Conversely in separating chroma from the composite video, if the bandwidth of the band-pass filter is wide, a portion of the luma signal will leak into the chroma signal, known in the art as false color; if the bandwidth of the band-pass filter is narrow, the chroma will lose sharpness.
In general, the bandwidths of the band-trap and band-pass filters for any particular SECAM application are static values based on a design compromise between sharpness and artifacts. As a result of these static values, the ability to separate the luma and chroma signals at various points in the frequency spectrum is reduced.